The Duke of Cambridge presented new Colours to the 1st Battalion Irish Guards ahead of the Queen's Platinum Jubilee celebrations next month.
The battalion will take part in the annual Trooping the Colour ceremony which marks the monarch's birthday, with the 2 June event forming part of the jubilee bank holiday weekend.
Prince William, who is the Colonel of the Irish Guards, was received on parade by 450 guardsmen and their families with a Royal salute before he inspected the front line of the regiment.
The new Colours were presented in a sacred ceremony on the Quadrangle at Windsor Castle.
Prince William wore his Irish Guards uniform as he presented the two new Colours, one of which – the new Queen's Colour – will be trooped by the regiment next month.
The Irish Guards will troop its Colour, an embroidered silk and gold flag, emblazoned with battle honours, on 2 June to mark the Platinum Jubilee.
New Colours are traditionally presented every 10 years, but the Irish Guards' old Colours date back to 2009.
The Colours presentation had been delayed by more than two years, partly as a result of the coronavirus pandemic, but also due to the Irish Guards' extensive operational commitments over recent years.
Watch: Irish Guards practise Trooping the Colour ahead of Platinum Jubilee celebrations.
Addressing the regiment during the ceremony, Prince William said: "In this historic Platinum Jubilee year, we rightly celebrate your many achievements.
"I know that Her Majesty is looking forward to seeing these new Colours trooped during her official birthday parade next month.
"I also know they will inspire you and your successors to emulate the deeds of the Irish Guardsmen who have gone before as you continue to serve our nation, in conflict or in peace, at home and abroad.
"The thoughts and prayers of my own family, which enjoys a close and proud connection with the Irish Guards, will be with you as you set about this most important task," he added.
Defence Secretary Ben Wallace, a former Officer of the Scots Guards, attended the ceremony, and the regiment's mascot, Irish wolfhound Turlough Mor – otherwise known as Seamus – also took part.
Watch: What are colours and why do they mean so much?
The mascot's handler, Drummer Adam Walsh, said: "Getting a new Colour, it happens only once or twice maybe in a guardsman's career, so it’s very special for me, especially with it being Seamus's first time as well.
"The two of us were very proud to be standing there while we were getting our new Colour from the colonel of the regiment."
He added that he was very pleased with Seamus's behaviour, saying: "He was so very still, there wasn't much moaning from him."
More than 1,500 soldiers and 350 horses from the Household Division will parade in front of 7,000 ticketed spectators for the Queen's Platinum Jubilee birthday parade in June.